(a). Field of the Invention
This invention relates to a resistor device, and more particularly, to a transistor resistor and an associated method.
(b). Description of the Prior Arts
Resistors are commonly used in integrated circuits (IC). Modern semiconductor processes offer several methods for resistor implementation. For example, the standard complimentary metal oxide semiconductor (CMOS) process provides several types of resistor, such as silicided polysilicon, silicided p+ or n+ active region, n-well and metal layer, where n-well has the highest resistance per unit area at about 1 k ohm/square. However, some circuits require very large resistance. For instance, a 3 dB 100 kHz first-order filter with 10 pF capacitance requires 160k ohm resistance, which means the resistor, even if a n-well resistor is used, will occupy a large space within the IC. This will create tremendous challenge in modern IC design where small-size, high-density IC is required.
Thus some IC designers would operate a MOS transistor in the resistive region (also called triode region) to use it as a resistor and obtain very high resistance per unit area. But a big drawback of such approach is that the resistance will change along with input signals, hence resulting in poor linearity.